Semiconductor device features are primarily fabricated using photolithography. The art of photolithography embodies techniques for creating two-dimensional patterns on a work surface, or target, by the controlled application of energy (such as electromagnetic, ion beam or other radiation) to a reactive material, or resist, deposited on the target. In a photolithographic process, the energy application is controlled through the use of a patterned photomask. The pattern is transferred to a resist coating on the target, forming a resist pattern. The target is then etched according to the resist pattern and, following the etch, subjected to further processing steps. In semiconductor fabrication, the target may be a semiconductor wafer and the resulting features form a portion of a final integrated circuit.
Typically, photolithography is achieved by projecting or transmitting energy through a pattern made of opaque areas and clear areas on a mask. In the case of optical photolithography, the opaque areas of the pattern block light, thereby casting shadows and creating dark areas, while the clear areas allow light to pass, thereby creating light areas. Energy is projected through the clear areas onto and through a lens and subsequently onto the target, such as a semiconductor wafer. The term opaque refers to any area that blocks a sufficient level of the projected energy such that any energy passing through the opaque area will produce only negligible reaction with the resist coating. The term clear refers to any area that permits a sufficient level of energy to project onto the target to react with the resist coating to produce a resist pattern. The resist pattern is used to protect portions of an underlying substrate during subsequent removal techniques, such as etching, to form a patterned layer on the substrate substantially duplicating the resist pattern.
In the process of forming a patterned layer through the use of a projection exposure, it is customary that a member used for reduced-size projection is termed a reticle, and a member for life-size projection is termed a mask; or a member corresponding to an original sheet is termed a reticle, and a member obtained by duplicating such a reticle is termed a mask. In the present invention, any of the masks and reticles classified by such various definitions are referred to as a mask for convenience. Furthermore, the term mask may also refer to a database representation used to produce a physical mask.
The process of producing a mask for an integrated circuit involves generating a composite drawing of the integrated circuit derived from a circuit layout, which is generated from the functional and schematic diagrams. The composite drawing represents the various layers of the integrated circuit, and each layer of the composite drawing will be used to generate a single mask. To transform a layer of the composite drawing into a mask, it is digitized. The resulting database representation defines the opaque and clear areas of the mask. The physical mask is typically produced by selectively establishing areas of opaque material, often a layer of chrome, on a clear support, often a glass or quartz plate. As will be apparent to the reader, areas of the clear support not covered by the opaque material are necessarily clear.
Because of increased semiconductor device complexity that results in increased pattern complexity, and increased pattern packing density on the mask, it is becoming increasingly difficult to produce a precise pattern image despite advances in photolithographic techniques. One problem leading to increased difficulty in transferring a pattern from a mask to the target is overlay error. Overlay error occurs where two discrete patterned layers are formed using masks on two separate lithography systems. Each lithography system will have a distortion fingerprint, i.e., imperfections in the lens and stepper mechanisms resulting in a translation or offset between the intended placement of an image feature and the actual placement of its projected image. Where one lithography system is used to produce a pattern on a first layer and a second lithography system is used to produce a pattern on a second layer, the differences in their distortion fingerprints result in a relative offset between corresponding features of the two patterns A common situation includes two lithography systems, with one having a larger field of exposure than the other. The exposure from the lithography system having the smaller field of exposure is reproduced usually two or more times in the field of exposure of the other lithography system. Another situation is simply where one exposure from one lithography system overlays an exposure from another lithography system.
Translation, rotation and magnification have been used to minimize overlay error. Despite such corrections for overlay error, features will still experience offset. This residual offset may be unacceptable as feature dimensions continue to decrease.
As can be seen, the accuracy of the mask pattern and the resulting resist pattern play important roles in the quality of the circuit. As feature size decreases, the impact of offset increases proportionately. As manufacturing requirements call for exposure of patterns with smaller and smaller dimensions, it is becoming necessary to employ techniques that permit enhancement of the current performance of the process of photolithography.